Equalization in a digital receiver is a process whereby multipath, noise, and other interferences incurred in the digital broadcast are removed from the received signal, attempting to restore the original digital transmission. Since the characteristics of the broadcast channel are rarely known a priori to the receiver, and can change dynamically, equalizers are usually implemented using adaptive filters.
Most state-of-the-art digital receivers use some type of decision feedback equalizer (DFE), because it provides superior inter-symbol interference (ISI) cancellation with less noise gain than a Finite Impulse Response (FIR)-only equalizer structure. Austin first proposed a DFE, in a report entitled “Decision feedback equalization for digital communication over dispersive channels,” MIT Lincoln Labs Technical Report No. 437, Lexington, Mass., August 1967. A DFE acts to additively cancel ISI by subtracting filtered symbol estimates from the received waveform.
Nearly all modem digital communication systems use some type of channel coding at the transmitter, and complementary decoding at the receiver. Channel coding typically induces some type of redundancy or overhead in the signal, at the expense of reduced bandwidth, with the benefit of better estimation of the transmitted signal. A common type of channel coding uses trellis coded modulation techniques; see, for example, chapter 3 of Trellis Coding, C. Schlegel, IEEE Press, NY, 1997.
Some prior art techniques combine equalization and decoding to provide better overall recovered signal error rates. For example, in “Delayed-decision feedback sequence estimation,” by A. Duel-Hallen and C. Heegard, in IEEE Transactions on Communications, vol. 37, no. 5, May 1989, a tunable detection algorithm is introduced for a contiguous block of symbols, where the length of the block is tunable, and the algorithm uses a reduced-state search which incorporates information from the feedback filter to calculate path metrics. The information and symbol estimates are constrained to be contiguous.
In “Reduced-state sequence estimation with set partitioning and decision feedback,” by M. Eyuboglu and S. Qureshi, in IEEE Transactions on Communications, vol. 36, no. 1, January 1988, a conventional viterbi algorithm is used to search a reduced-state trellis, constructed using set partitioning, so that the complexity of the maximum likelihood approach is reduced, with little loss of performance.
In “Block decision feedback equalization,” by D. Williamson et al., in IEEE Transactions on Communications, vol. 40, no. 2, February 1992, a generalization of the DFE is presented where a contiguous block of data is used to estimate a contiguous block of symbols. The algorithm is tunable in the block-length of data used, and the block-length of symbols estimated, and is shown to be a generalization of the maximum likelihood sequence estimator and the maximum symbol-by-symbol a posteriori detector.
In “Decision feedback equalization with trellis decoding,” by R. Gitlin and N. Zervos, in U.S. Pat. No. 5,056,117, Oct. 8, 1991, a trellis decoder is used to provide tentative decisions derived from survival paths of the Viterbi algorithm to the feedback filter in the DFE so as to minimize feedback errors.
All of the aforementioned publications are hereby incorporated by reference.
The present invention uses joint trellis decoding and decision feedback equalization to efficiently estimate non-contiguous symbols using equalizer outputs that are also not contiguous. The estimation process uses all new possibilities of symbol values, instead of old decision feedback symbol estimates.